1. Field of the Invention
The present invention relates to an improved process for preparing aldehydes by reacting olefins with carbon monoxide and hydrogen in the presence of rhodium carbonyl compounds containing phosphines as complex ligands.
2. Discussion of Prior Art
The hydroformylation of olefins to aldehyehydes takes place in a substantially more selective manner with rhodium catalysts than with cobalt catalysts. It is thus possible to carry out the oxo synthesis on a technical scale with very small concentrations of rhodium as catalyst. Nevertheless, the economy of this variation of the oxo process is substantially influenced by the use of rhodium since rhodium is about one thousand times more expensive than cobalt. When using rhodium catalysts an effort is therefore made not only to convert the olefin employed to aldehyde, without the formation of by-products, but also to restrict the consumption of catalyst to an absolute minimum.
The selectivity of the hydroformylation in the presence of rhodium catalysts can be improved as regards straight chain aldehydes by using rhodium carbonyls that contain phosphorus ligands. Thus, for example, German Offenlegungsschrift No. 19 39 322 describes the hydroformylation of olefinically unsaturated compounds with hydridocarbonyl-bis (trisubstituted phosphine)-rhodium or hydridocarbonyl-tris(trisubstituted phosphine)-rhodium compounds as catalyst. Trisubstituted phosphines include, inter alia, those containing aryl, alkyl or aralkyl radicals or substituted aryl, alkyl or aralkyl radicals, and phosphites are also mentioned. A basic feature of this process is that the olefinically unsaturated compounds are converted in the liquid phase, in which the reaction products remain after the conversion.
Since the reaction product impairs the activity of the catalyst solution, efforts are made to separate the reaction products and catalyst from one another as quickly as possible.
According to a known procedure, a heterogeneous catalyst system is employed in which the actual catalyst is bonded so firmly to a carrier that it cannot be removed therefrom under the reaction conditions. If the starting substances are brought into contact with the firmly arranged catalyst, the olefin is converted into the catalyst-free reaction product.
Other processes use rhodium carbonyl compounds containing complex ligands as catalyst, which are dissolved in a solvent having a very low vapor pressure. When the gaseous feedstock is passed through the catalyst solution the olefin reacts in the solution with the synthesis gas and the hydroformylation products that are formed, which are liquid per se, are immediately expelled together with the excess gases from the catalyst solution. Such processes are described for example in German Offenlegungsschrift No. 17 68 303 and in U.K. Patent Specification No. 13 87 657.
In all the afore-mentioned hydroformylation processes, in which a catalyst solution consisting of complexed rhodium carbonyls is used, the main problem is the decrease in the activity of the catalyst solutions, which are either recycled or kept stationary in the reactor. As a result of this decrease in activity a proportion of the catalyst solution must either constantly be replaced or the whole catalyst solution must be replaced after a certain time by a new, active solution. The processing of a spent catalyst solution consisting essentially of triphenylphosphine and high molecular weight hydroformylation products is not a simple process on account of the very valuable rhodium contained in the solution. A whole series of methods have in fact been described, according to which it is possible to recover the active Rh catalyst from residues of the oxo synthesis with rhodium-complex catalysts. However, the technical useful effect of this process is very slight and accordingly this process has not achieved any practical significance. In order to restore the full activity of the rhodium catalyst it is necessary to reconvert the rhodium into a pure inorganic rhodium salt, with the removal of all organic constituents of the catalyst solution. Since rhodium losses must be avoided, such procedure can be economically carried out only by means of precious metal separators. This means that a certain amount of rhodium catalyst material is always in the process of being shuttled backwards and forwards for processing between the companies employing the oxo process, and the precious metal processors. In addition to the cost arising from the processing and from the associated rhodium losses, the amount of rhodium being processed at any time represents an additional investment. For this reason it is desirable to prolong the life of the complex rhodium-carbonyl catalysts as long as possible by maintaining a constant activity.